In capacitive sensor devices, a capacitor arrangement is created and structured so that circuitry attached to the capacitor plates can sense a change in capacitance Such a change can occur, for example, when one plate of the capacitor is pressed toward an opposite plate by an externally (directly or indirectly) applied touch force, or as a result of a human hand altering the electrical field of the capacitor through interaction with the fringing fields around the plates. Additionally, the presence of a human finger or similar object may be sensed even without an actual touching, when it is in close proximity to one plate of one or more capacitors, as the electrical field—and hence the capacitance—of the arrangement is altered by the presence of the finger or other object. The mere proximity of a portion of the human body, such as a finger, can create a sufficient variation in capacitance, without the need to apply pressure to a plate so as to move the plates closer together, to permit sensing of that condition.
Unfortunately, this sensitivity to approaching fingers is problematic. It is well known that capacitive sensors also are very sensitive to capacitance variability due to environmental changes, such as humidity, temperature, dirt, and so forth. A capacitive sensing system must, therefore, be able to distinguish reliably between capacitance changes that are due to environmental changes and capacitance changes that are due to an operator actually touching or approaching the sensor. It is undesirable to allow environmental changes to produce an output that could be interpreted as a touch or near touch. Conversely, it is also undesirable that a hand hovering in the vicinity of the sensor be interpreted as an environmental condition, for doing so may cause an environmental compensation process to change the sensitivity profile of the sensor and render it insensitive to an actual touch.
In the example of FIG. 1, there is shown diagrammatically the construction and operation of a capacitive sensor, for tutorial purposes. Reference will be made to this diagram in aid of explaining how environmental factors and a “hovering” hand can complicate the operation of such sensors, reducing their sensitivity and sometimes doing so to the point of rendering a sensor unresponsive to a user's touch. A differential construction is shown, with two capacitors 10A and 10B sharing one plate 12 in common, but it will be appreciated that single-ended designs may be substituted. Common plate 12 is connected to receive a transmit signal 14 on line 15 from a signal source (not shown). Opposing the common plate 12 are second and third plates 16A and 16B, each of which forms the second plate of a capacitor with common plate 12. In turn, these two plates 16A and 16B are connected, respectively, to the non-inverting and inverting inputs of a differential capacitance-to-digital converter (CDC), such as 18, having, for example, a 16-bit output. The plates (electrodes) 12 and 16 may typically be arranged so that a portion of the electric field between them, represented by lines 22, arcs into the region above or in front of the electrodes; most of the field, though, is directly between the electrodes, as indicated by field lines 24. When a user's hand approaches the electrodes, it resembles to the circuit, electrically speaking, a large “ground” mass, and reduces the field intensity between the transmitter and receiver electrodes 12, 16A and 16B, respectively. This alters the capacitance of the capacitor. The output value, or code, of CDC 18 is indicative of the close position of the ground mass (e.g., hand). Naturally, comparable single-ended arrangements are known, as well.
Now, referring to FIG. 2, assume that a user first presses down with a finger on the left-hand capacitor, which we will call sensor 16A, and then withdraws that finger and then puts it on the right-hand capacitor, which we will call sensor 16B. FIG. 2 shows the ideal output 30 of the CDC. Prior to the first sensor press, between sensor presses and after the second sensor press, the CDC output is stable at a fairly constant ambient value indicated within dotted rectangles 42. When the user's finger presses on sensor 16A, the output CDC value rises to a maximum output CDCmax and when the user's finger is put on sensor 16B, the CDC output falls to a minimum value CDCmin. The output value of the CDC is read by a processor (not shown). Typically, the processor uses a first threshold value 48 to determine when the output of the CDC indicates that the first sensor has been pressed and a second threshold value 52 to indicate when the second sensor has been pressed. When a threshold is crossed, an interrupt is generated. Real-world operation, however, rarely produces such idealized operation. Rather, when the environment changes, the ambient value of the CDC output drifts. With stationary thresholds, this would be quite problematic, as illustrated by the CDC value waveform 30′ in FIG. 3. There, ambient drift is indicated in the changes in ambient levels of waveform 30′ from 54 to 56 to 58. As a consequence, though a finger on sensor A is still detected (i.e., threshold 48 is crossed), a finger on sensor B does not cause the CDC output value to cross the threshold 52, so no interrupt is generated and the processor never detects that the second sensor was pressed.
This problem can be overcome if the ambient value of the CDC output changes due only to environmental factors and the threshold values can be made to change with changes in the ambient CDC output. (The “ambient” value of the CDC output to be tracked is the background value of the CDC output with all human hands kept distant so as not to touch the sensors. This ambient value is calculated from the CDC output when the user is not close to or touching a sensor.) More particularly, it would be desired that the threshold values remain at equal distance from the (moving) ambient value while tracking or calibrating for environmental changes, but this ideal often is not achieved. FIG. 4 shows at 62 and 64 the situation that would result with the threshold values 48 and 52 changing to track the ambient drift. That is, both sensor presses generate interrupts.
While various techniques exist to compensate for ambient drift in capacitive sensing arrangements, these approaches have not proven to be ideal and, indeed, some will still permit environmentally-induced drift to allow input devices to become temporarily insensitive to touch. For example, with some systems, if a user's finger hovers in the vicinity of a key or input button, but does not press the key or button, the hovering presence of the user's body is interpreted as a sign of drift that requires compensation, and the resulting over-compensation may render the key or button actually insensitive to touch for a period of time.